What is the best export format from AutoCAD -> Blender?
I have always found FBX to be the most reliable way to export from Autodesk products for use in Blender. It tends to keep the correct dimensions, UVs, and Blender can estimate the materials quite well (albeit with some junk nodes thrown in for maximum compatibility).

That being said, it seems Autodesk has been removing FBX exporting from AutoCAD, so I don't know if that will be the best option for you if are using or plan to use AutoCAD 2019. I have heard from an interview with Ton Roosendaal that Autodesk has been removing FBX because it has become too accessible to 3rd parties, and so they can't make as much money leaving it available in their software. A shame, if true, but there could be many other reasons we aren't privy to.